Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, a flow rate or amount of EGR may be adjusted via an EGR valve disposed in the EGR passage. However, adjusting the EGR valve alone may not adequately control EGR to a desired level. For example, if the EGR valve is already in a fully open position and the desired EGR level is higher than a currently delivered EGR level, the EGR valve cannot be adjusted to further increase EGR above the current level. Additionally, closing the EGR valve in response to a request to decrease EGR flow may increase pressures within the scavenge manifold and/or result in increased residual gases within the cylinders due to decreased blowthrough air to the intake passage.
In one example, the issues described above may be addressed by a method, comprising: adjusting a timing of a first set of cylinder exhaust valves to adjust a flow of exhaust from engine cylinders to an intake passage, upstream of a compressor, via an exhaust gas recirculation (EGR) passage, where the first set of cylinder exhaust valves open at a different time than a second set of cylinder exhaust valves coupled to an exhaust passage. In this way, the flow of exhaust from the engine cylinders to the intake passage (e.g., EGR flow) may be adjusted by adjusting the timing of the first set of cylinder exhaust valves and not the EGR valve, thereby delivering a desired EGR flow while maintaining a desired blowthrough amount and increasing engine efficiency.
As one example, advancing the timing of the first set of cylinder exhaust valves may increase the EGR flow through an EGR passage coupled between the intake passage and a first exhaust manifold coupled to the first set of cylinder exhaust valves. In another example, retarding the timing of the first set of cylinder exhaust valves may decrease the EGR flow. While EGR flow is being adjusted to a desired level via adjusting the timing of the first set of exhaust valves, an EGR valve disposed in the EGR passage may be maintained in a fully open position. Controlling EGR flow in this way may provide a more consistent EGR flow where a fixed amount of EGR is pushed to the intake passage in each engine cycle. Further, controlling the EGR flow in this way may allow the EGR valve to be an on/off valve, thereby simplifying EGR valve control and reducing engine system costs.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.